Add basic documentation (#14)

* Docs MVP

* Change to mintlify

* Add more docs

* Init docs

apps/docs/introduction.mdx

@@ -0,0 +1,33 @@
+---
+title: Introduction
+description: "Unsend is Open source alternative to Resend, Sendgrid, Mailgun and Postmark etc."
+---
+
+## Setting up
+
+Quicklinks to set up your account and get started
+
+<CardGroup col={2}>
+  <Card
+    title="Add domain"
+    icon="globe"
+    href="https://app.unsend.dev/domains"
+  >
+  Add domains to send emails
+  </Card>
+
+  <Card
+    title="Create API key"
+    icon="key"
+    href="https://app.unsend.dev/api-keys"
+  >
+    Generate API key to send emails from your app.
+  </Card>
+  <Card
+    title="Try our API"
+    icon="code"
+    href="/api-reference/introduction"
+  >
+    Learn how to use our API to send emails programmatically.
+  </Card>
+</CardGroup>